In 2017 Williams began appearing in the Starz science-fiction series, Counterpart, playing Emily, the (ex)wife of lead character Howard Silk. In one universe she is still married to him, but her counterpart in the other universe is divorced from him.

In 2000, Williams wrote the short story "The Significance of Hair" for BBC Radio, and read it on the air.[8]

Williams had a seven-year relationship and then engagement to the actor Jonathan Cake which ended two weeks before their planned wedding. In 2003, she married the actor and playwright Rhashan Stone, with whom she has two daughters.[9]